1. Technical Field
The present invention relates to a method and system for controlling a voltage of a fuel cell in a stop-mode of a fuel cell vehicle capable of securing durability of the fuel cell and preventing fuel efficiency from reducing by actively decreasing a voltage when the fuel cell of the fuel cell vehicle stops.
2. Description of the Related Art
In a fuel cell vehicle, when a stop entry condition of a fuel cell is satisfied, a stop-mode process is performed. The stop-mode process suffers from a power generation stopping process that stops an air blower to confirm whether an air flow rate is reduced. When the air flow rate is reduced, the power generation stopping process is completed and the fuel cell vehicle enters the stop mode.
Even after the fuel cell enters the stop mode, a voltage of a fuel cell stack does not immediately decrease. When ram air is introduced or no hydrogen cross over is present, the voltage is continuously maintained near an open-circuit voltage (OCV), which leads to a deterioration in durability of the stack. Further, there is a need to forcibly decrease the stack voltage, but when the stack voltage is maintained to be too low, a reacceleration deteriorates due to time required for voltage increase when the fuel cell restarts. Therefore, there is a need to control the voltage decrease within a range where the reacceleration of the vehicle does not deteriorate to prevent the stack voltage from being maintained near the OCV.
Meanwhile, a resistor may be connected to decrease the voltage, which leads to the reduction in fuel efficiency. Therefore, a method for generating an output of the fuel cell by a control of a voltage decrease of a bus terminal using a power converter to decrease a voltage of the fuel cell (e.g., when a supply of air stops), consuming the output of the fuel cell by a driving motor or high voltage auxiliaries, and forcibly charging extra power with a high voltage battery is used.
The output of the fuel cell may be generated by the control of the voltage decrease of the bus terminal using a power converter of a high voltage battery of the vehicle and since an available output for charging a battery is limited, when a regenerative braking amount is substantial, regenerative braking energy is reduced, which may cause a loss of fuel efficiency. Further, when the generation of the output of the regenerative braking and the output of the fuel cell is not reduced, the battery may be overcharged.
Therefore, the related art controls the voltage decrease of the bus terminal using the power converter only when there is no vehicle speed or the current regenerative braking amount is not greater than the limited value of the regenerative braking, by preventing the battery from being overcharged and considering the fuel efficiency. Further, a process of determining whether the high voltage battery is in a chargeable state may be added. In other words, the voltage decreasing control of the bus terminal to generate the output of the fuel cell and decrease the voltage of the fuel cell is performed by understanding in advance the situation that the loss of the fuel efficiency and the overcharging of the battery may be prevented.
However, when the output of the fuel cell is unable to be generated by the control of the voltage decrease of the bus terminal by determining in advance based on the vehicle speed, a margin of the regenerative braking, and whether the battery is in a chargeable state, etc., the control of the voltage decrease of the bus terminal may not be performed even when a minimal amount of output of the fuel cell may be generated. However, insufficient the margin of the regenerative braking is, the minimal of output of the fuel cell as much as the insufficient margin may be generated and thus, the voltage of the fuel cell may decrease even at a slow speed. Therefore, according to the related art, the voltage of the fuel cell decreases at a slow speed or does not sufficiently decrease, and thus the durability of the fuel cell may not be sufficiently secured.
The contents described as the related art have been provided only for assisting in the understanding for the background of the present invention and should not be considered as corresponding to the related art known to those skilled in the art.